Fluent Interfaces with ‘with’

An ad-hoc embedded domain specific language, aka Fluent Interface, is an API that reads like an English sentence. LINQ does an excellent job of implementing this style so you can write long queries in normal dot notation rather than the query syntax. Someone at Google posted a collection library for Java that does something similar to LINQ, but it’s not as readable as LINQ’s extension methods. Someone wrapped the code with a fluent interface (from google cache) to make it more readable. This requires that all the methods return an Iterables interface so you can do proper method chaining. It doesn’t fit the normal OO-style, and can be tedious to wrap existing classes into this style.

A minor bit of syntactic sugar that I once thought would be useful for C# and Java is the ‘with’ expression, which allows you to write code like this:

with person {
    .FirstName = "Bob";
    .LastName = "Smith";

Within the body of the ‘with’, any illegal use of the dot notation (“.Member”) is assumed to use the object “person”. The syntax is just “with Expr Body”, and it returns the Expr when it’s done. With this syntactic sugar, you can now write normal OO code (stateful changes on the same object) that reads like a Fluent Interface. It turns out that this syntax also subsumes the object initializer syntax in C#. Compare these two lines:

Person value1 = new Person { Name = "Chris Smith", Age = 31, CanCode = false};
Person value2 = with new Person() { .Name = "Chris Smith"; .Age = 31; .CanCode = false; };

Line #2 is only a few characters longer, but the syntax is more general than the object initializer in line #1. I can insert arbitrary code in the body, not just initialization code. And I can use the ‘with’ expression to write fluent code on existing OO APIs as shown above. Basically, if C#’s object initializer is a good idea, then my ‘with’ expression is a better idea, IMHO.



  1. Dave

    That’s not what I call a fluent interface!
    If your with statement could make ‘new Person.SetName(“Chris Smith”).SetAge(31).CanCode(false)’ work that I would be a better idea.

    Now it’s just another initializer and in my opinion that has nothing todo with fluent interfaces. Maybe it’s wise that you take a look at projects like FluentHibernate, Ninject or Window Container 2.0 to get a grasp with fluent interfacing really is (or atleast what we call fluent).

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